4-link geometry question

[Bjkadron]

Hi everybody! I'm currently building up a Plymouth Duster and I'm trying to figure out what rear suspension design to use. But this specific question is about 4-link geometry.

What are the differences, Geometry wise, between a drag racing style 4-link and a road racing one? Aside from the different axle locating parts such as triangulation, panhard rod, and watts link? I know the roll center, Roll steer, and the anti-squat are different(and more important) but other that that? What are the differences?

Thanks for the help!

[BillyShope]

I don't know what's concerning you beyond roll center, roll steer, and percent antisquat.

Dragracers don't worry too much about roll center location, so long as they can keep it between the guard rails.

Nor are they concerned with roll steer, for the same reason. The road racer should be concerned with roll steer only in so much as it affects the comfort of the driver. While the terms "roll understeer" and "roll oversteer" are commonly used, it must be remembered that these terms relate only to the change in steering angle and are not associated with the dynamics involved with under/oversteer when such behavior is associated with, for instance, roll center locations. In other words, a car with roll oversteer has a critical speed only if the oversteer persists when the roll oversteer is removed.

The dragracer prefers antisquat values equal to, or above, 100%. To avoid wheel hop problems under braking, the road racer usually prefers values well into the "squat" region.

Personally, I value the KISS principle and would therefore opt for a 3link with Panhard (with the "odd" link offset to the right to counteract driveshaft torque effects). My site has a spreadsheet for 3link setup.

http://home.earthlink.net/~whshope

[Bjkadron]

Thanks for the reply!

I was more concerned about the contrast between the two... Such as "is there really any difference?"

Thanks for the site! I'll check it out!

[Bjkadron]

Ooops! wrong button!

I was mostly wondering if i could use the 4-link design off the "mopar missile" super stock cars for road racing in my Duster(maybe triangulating the lower arms for a satchel link design or panhard bar maybe). Because I know a lot of time and effort went into designing it from people a lot more knowledgeable than me!

[rjsjea]

Reilly motorsports makes a triangulated kit specific for your car. If it is information you want, call Bill (RMS) he will give you the low down.

https://www.reillymotorsports.com/ga...&cat=0&pos=-80

[Bjkadron]

Thanks for all the info but I Am thinking that it would be fun to build my own. Also that way there would not be a bunch of others out there with something just like it.

[rjsjea]

Thanks for all the info but I Am thinking that it would be fun to build my own. Also that way there would not be a bunch of others out there with something just like it.

I'm with ya......I didnt use that kit for my Demon, just an easy option.

[David Pozzi]

Below are a couple of shots of a Trans-Am race car. Nearly every car in that series had this type of 3 link suspension. the front end of the odd link has an easy way to adjust height.
David


Note the offset of the odd link to the passenger side.

[Bjkadron]

WOW! Great pictures! That Trans-am racing suspension looks like exactly what I'm looking for! Do you know where I could get more info/pictures and possibly measurements? and that 4-link on the Demon looks cool too! Any pics/info available on that?

[BillyShope]

WOW! Great pictures! That Trans-am racing suspension looks like exactly what I'm looking for! Do you know where I could get more info/pictures and possibly measurements? and that 4-link on the Demon looks cool too! Any pics/info available on that?

You ask for measurements. You don't want to copy something from another car, for link angles and mounting points are car specific. Again, my site provides that which will work for your car.

http://home.earthlink.net/~whshope

[Norm Peterson]

Roll steer

Too much of that (in the vehicle understeer direction) tends to make a car feel "clumsy" during slalom or lane-change maneuvers.


Norm

[Bjkadron]

You ask for measurements. You don't want to copy something from another car, for link angles and mounting points are car specific. Again, my site provides that which will work for your car.

http://home.earthlink.net/~whshope

I know I don't want to copy something from another car But I'm still slightly confused what to do with the calculator(I'm assuming you meant page 18). First off, when you say offset to the right I'm assuming you mean to the passenger side? Also you said that the odd link could be top or bottom, But how do I figure out which is better for me? Is the 20" link length best or do I just figure out what fits best? Does the rear mounting point have to be in front of the axle? And I have no Idea how to figure out my Center of gravity!
Should I just use the stuff that is already there? Also I'm still in the designing phase so I don't even know what my car weighs or what tires and rear gear I'm going to use. I 'm just trying to get everything figured out on paper so I have it all right when I build it. But I was planning to make the link mounting points adjustable so I could adjust to different things. Also I'm still not sure how much anti-squat I need until I go to the track and try it, But do you have a suggestion for a good starting point? I know I have a lot of questions so if you don't want to answer them then I don't blame you.

Thanks again for all the info!

[David Pozzi]

Sorry I don't have any more photos, I took a bunch but they were all lost when my hard drive crashed a while back.

[Norm Peterson]

Also I'm still in the designing phase so I don't even know what my car weighs or what tires and rear gear I'm going to use. I 'm just trying to get everything figured out on paper so I have it all right when I build it. But I was planning to make the link mounting points adjustable so I could adjust to different things. Also I'm still not sure how much anti-squat I need until I go to the track and try it, But do you have a suggestion for a good starting point?

Start by defining what the car's mission in life is going to be, and what you want it to do better. Put these in some sort of order of importance to you. And consider what sort of compromises you're willing to make (or not). Not just the straight line vs cornering decisions, but if (for example) you want (need?) 4 person capacity, the 3-link in David's picture probably won't make the cut.

Billy's suggestion of a 3-link (plus PHB or Watts link) deserves further consideration. It separates the issues of RCH and roll steer from anti-squat much better than a 4-link is capable of doing. A torque arm/PHB such as is under the 3rd/4th gen F-bodies can be a reasonable alternative.

I suggest not making everything wildly adjustable to suit any possibility for what I assume will remain mostly a street-driven car.


Norm

[Bjkadron]

Start by defining what the car's mission in life is going to be, and what you want it to do better. Put these in some sort of order of importance to you. And consider what sort of compromises you're willing to make (or not). Not just the straight line vs cornering decisions, but if (for example) you want (need?) 4 person capacity, the 3-link in David's picture probably won't make the cut.

Billy's suggestion of a 3-link (plus PHB or Watts link) deserves further consideration. It separates the issues of RCH and roll steer from anti-squat much better than a 4-link is capable of doing. A torque arm/PHB such as is under the 3rd/4th gen F-bodies can be a reasonable alternative.

I suggest not making everything wildly adjustable to suit any possibility for what I assume will remain mostly a street-driven car.


Norm

The car is going to have multiple missions the first one is to get me to a tech school(when It will be mostly stock and my daily driver). The other one that I'm doing all this planning for is the "halo" car for my future custom automotive business and my eventual goal is for it to be the "fastest street legal road racing Duster ever" Mostly I want it to be really fast on a road course. Because I want to do a lot of open road racing, AutoXing and to tear up all those import guys at time attack events, as well as maybe a little drifting for fun and the occasional (stress on "occasional) trip down the strip.I also wanted to do standing mile racing but you don't need real hard launches for that. I'll never put drag slicks on it so the tires will probably spin and not put a lot of stress on the driveline and suspension. So your right a 3-link W/PHB will probably work better for me(And I like them better than 4-links anyway). The pictures David posted are probably about what my car will eventually look like. Also the engine combo's will range from a mostly stock 360(start) to a 700-800 hp screamer(completed) Like a used NASCAR engine or something like that, So thats why I wanted to do the adjustability so it could "grow" with me. And the balance will be leaning more toward the "race" end of the scale rather than the "street" end. So I'd be more willing to compromise on things like ride quality and drag starts than turning corners.

Thanks again!
Benjamin

[Bjkadron]

I just wanted to thank you all again for all the information and help! I really appreciate it!

Benjamin